Combined Sewer Overflow (CSO)

Combined Sewer Overflow (CSO) occurs when heavy rainfall or snowmelt causes a city's sewer system—designed to carry both sewage and stormwater—to exceed its capacity. As a result, untreated wastewater and stormwater are temporarily released directly into nearby water bodies, such as rivers or lakes. This process helps prevent sewer backups into homes and streets but can introduce pollutants and contaminants into the environment, affecting water quality and ecosystems. CSOs are managed through infrastructure improvements and monitoring to reduce their frequency and impact on public health and the environment.